1. Field of the Invention
The present invention relates to a heat accumulation correcting method, a thermal printer, and a computer-executable program. More particularly, the present invention relates to a heat accumulation correcting method used with a thermal printer for preventing an image quality from being degraded by accumulated heat of a heating element, and a thermal printer, and a computer-executable program.
2. Description Related to the Prior Art
There are a direct thermal recording type of thermal printer and a thermal transfer recording type of thermal printer. The direct thermal recording heats a thermosensitive recording material with a thermal head to directly develop color. The thermal transfer recording heats the back side of an ink ribbon placed upon a recording material to transfer ink in the ink ribbon to a recording material. A thermal head has a number of heating elements disposed on a ceramic board.
It is general in a thermal printer that no image can be recorded with high fidelity if the thermal head is driven directly according to input image data. This is because irregularity in density or unsharp state of an image contour is likely to occur in a printed image due to heat accumulation.
Part of the heat energy accumulated in a heating element contributes to recording a pixel. However, remaining heat energy short of developing color may be dissipated or accumulated in a glaze layer where the heating elements are located. Heat energy accumulated in the glaze layer may be conducted to a ceramic board for supporting the glaze layer, and may remain therein. Also, heat energy accumulated in the ceramic board may be conducted to the aluminum panel for supporting the ceramic board, and may remain therein, and also may be conducted to a plate of a heat sink secured to the aluminum panel, and may dissipate. It is likely that part of accumulated heat in any of the glaze layer, the ceramic board and the aluminum panel in the thermal head conducts back to the heating elements, and causes unwanted influence to the recording of a succeeding line.
Part of accumulated heat in the layers of the thermal head influences recording of pixels. Therefore, color developing density may be higher than an expected level. If one first portion on an original image has shape in with a distinct change in the density from a dark color to a light color, the first portion viewed on an obtained hard copy is likely to have an unsharp change in the density. Recording of the contours is impossible in a sharp manner. Also, the heat accumulation causes a phenomenon of shading, in which density is rather low at the start of the printing, and density will raise in the course of the printing due to heat accumulation within the heating elements. The farther the recording proceeds, the more the amount of accumulated heat in the heating elements, to cause the shading.
Ideas to prevent drop in the image quality according to the heat accumulation correction are disclosed in U.S. Pat. No. 5,539,443 (corresponding to JP-A 6-015863), U.S. Pat. No. 5,841,461 (corresponding to JP-A 9-052382), JP-A 7-223334, JP-A 2000-071506, JP-A 2000-108399 and JP-A 2002-166588. A thermal printer of those documents has a thermal head including a layer of heat accumulation, and in which a function of first order delay, a function of second order delay, a linear function or other suitable methods are used for the purpose.
A printing speed is an important factor in performance of a thermal printer or other types of printers, and specifically for commercial use to sell prints as products. Correcting methods with higher precision than before are suggested in further improved techniques, for example JP-A 10-146998 and U.S. Pat. No. 6,494,629 (corresponding to JP-A 2001-270144). An algorithm for the correction is in consideration of a use of plural heat accumulating layers. The layers are elements combined together for constituting a thermal head. Those are multi step correction of heat accumulation as a new conception over single step correction of heat accumulation so far suggested.
Although thermal history correction of a multi step type has higher precision than that of a single step type, there remains a problem in that precision of the correction near to the rear edge of the printing region is lower in a large size print than the L-size print. Examples of the large size print is 2L or 3L having greater areas than the L size. This is because their size in the sub scan direction is greater than the of the L size.